Electrical isolation of opto-electronic device components

ABSTRACT

The present invention relates to the electrical isolation of components within an integrated opto-electronic device where two or more active regions are optically coupled, for example by a waveguide. The device includes a distributed feed-back laser diode and an electro-absorption modulator fabricated on the same substrate. The laser diode and modulator are: separated by an electrical isolation region; linked optically across the isolation region by a waveguide; and capped by a ternary cap layer through which ohmic contacts are made to operate the components. The cap layer extends to the isolation region from which a grounding contact is made to ground the cap layer in the isolation region and so to electrically isolate the laser diode and modulator from each other.

The present invention relates generally to the electrical isolation ofcomponents within an integrated opto-electronic device where two or moreactive regions are optically coupled, for example by a waveguide. Theinvention particularly relates to isolation of a distributed feed-back(DFB) laser diode and an electro-absorption (EA) modulator in amonolithically integrated opto-electronic transmitter device for afibre-optic telecommunications link.

Opto-electronic integration gives the potential for low cost, reliableand compact components, improved temperature and mechanical stability,and assured alignment between components. In the field of transmitterdevices for fibre-optic communications, the integration of a laser diodewith a modulator, for example in a buried heterostructure or a ridgestripe, was achieved many years ago, for example see the publication byS. Tarucha and H. Okamoto in Appl. Phys. Lett., vol. 22, pp. 242-243,1986 relating to a device fabricated from GaAs. Nowadays, in order toachieve operation at 1.55 μm, such integrated opto-electronictransmitter devices are usually fabricated from on a wafer grown from ann⁺⁺-InP substrate on which are grown a number of layers, including ap⁺-InP active layer capped by a p⁺⁺-GaInAs ternary or cap layer. The caplayer has a relatively low resistance, and so serves as a contact layerto which electrical contacts may be made.

Ideally, the light output from the laser diode should have a steadywavelength and intensity. However, because of the integrated nature ofsuch structures, and close physical proximity of the components, theelectrical resistance between the laser diode and modulator (referred toherein as the isolation resistance) will be about 1-10 kΩ, depending onthe conductivity of the p-contact material and the separation of thecontacts. It is therefore possible that the electrical signal used tomodulate the modulator can inadvertently affect the laser diode, causingwavelength and/or intensity shifts. The isolation resistance thereforeneeds to be increased, at least to a few 100 kΩ, and preferably to a fewMΩ.

Several approaches have been suggested to deal with this problem ofelectrical isolation. One approach is to etch through the ternary caplayer. This increases the isolation resistance only to about 10-20 kΩ.Although such etching does not interfere with the optical waveguidebetween the laser diode and the modulator, this is insufficientisolation.

One way to achieve sufficient isolation is disclosed in the publicationby M. Suzuki et al in the Journal of Lightwave Technology, vol. 6, pp.779-785. An isolation region is formed in the stripe, between the DFBlaser diode and the EA modulator, by etching away both the cap andactive layers. The gap in the active layer was then filled with apassivating SiN film and a polyimide. Although a relatively highisolation resistance of 2.5 MΩ was achieved, this approach suffers fromthe disadvantage of cutting into the optical waveguide between the laserdiode and modulator, which may adversely affect the optical efficiencyof the device or reduce coupling between the components owing tounwanted internal reflections.

Another way to achieve a high degree of electrical isolation, withoutetching away the cap and active layers and without adversely affectingthe optical performance of the device, is to use deep protonimplantation in the region between the laser diode and the modulator. M.Aoki and H. Sano have reporting in “OFC '95 Optical Fibre Communication,Summaries of Papers Presented at the Conference on Optical FibreCommunication, vol. 8, pp. 25-26, pub. Optical Society of America 1995”,that this technique can achieve an electrical isolation of greater than1 MΩ. It is believed that this approach could achieve an isolationresistance of up to 10 MΩ. However, none of the other process stepsassociated with the fabrication of such an integrated opto-electronicdevice require any such proton or ion implantation, and so this approachrequires an investment in an additional item of very expensiveproduction equipment.

According to the invention, there is provided an integratedopto-electronic device, comprising at least two opto-electroniccomponents fabricated on the same substrate, two of the componentsbeing: separated by an electrical isolation region; linked opticallyacross the isolation region by a waveguide; and capped by a contactlayer through which ohmic contacts are made to operate the components,characterised in that the contact layer extends to the isolation regionfrom which a grounding contact is made to ground the contact layer inthe isolation region and so electrically isolate the two components fromeach other.

The grounding contact may then draw any stray currents from one of saidtwo components which could adversely affect the performance of the otherof said component. In the case of the component being a laser diode, andthe other component being a modulator, for example an EA modulator, formodulating the output of the laser diode, stray modulation currents atthe modulation frequency may be drawn to the grounding contact in orderto prevent wavelength or intensity chirping of the otherwise steadilybiased laser diode.

The contact layer may be broken or cut through, at least partially, inorder to help block current flow from one component to anothercomponent. Preferably, however, the contact layer extends contiguouslyin the isolation region between said two components.

In a preferred embodiment of the invention, a passivating layer coversthe cap layer in order to provide environmental protection for thedevice. The passivating layer may then have contact windows in thepassivating layer through which the contacts are made.

In general, the integrated device will have a ground plane, for exampleeither the substrate or a layer grown on the substrate. In many cases,the device will be fabricated so that the substrate is a ground plane.The grounding contact from the cap layer may then be made to the groundplane. If a contact window is provided to the substrate, then thegrounding contact may conveniently be made through this window to theground plane. It would however, be possible for the grounding contact tobe made, for example by a wire, or some other suitable grounding point.The ground plane need not necessarily be a zero volts with respect tothe earth, but will be at a suitable potential with respect to thecomponents in order to draw stray currents away from one or more of thecomponents. However, it may be the case that at least one of thecomponents is grounded by the ground plane.

It may often be the case that a passivating layer extends over thedevice, and in a preferred embodiment of the invention, this devicepassivating layer is contiguous with the cap passivating layer. Thewindow to the substrate may then extend through the device passivatinglayer in order to provide a convenient route to the ground plane.

It may be convenient to fabricate one or each of the contacts bydepositing a conducting layer, rather than by using wires. Thisdeposited conducting layer may then cover over one or more windows.Preferably the contacts are deposited so that the contacts cover overfully all of the contact windows. The deposited conducting layer(s) maythen act as a type of passivating layer in the area of the windows nototherwise protected by the aforementioned passivating layer.

It may be desirable to limit the stray currents drawn by the groundingcontact, for example in order to prevent overheating due to ohmiclosses, or to avoid a voltage drop for the components. Therefore, aresistance may be provided in line with the current path to ground. Atleast part of this resistance may conveniently be provided by the caplayer, which may be fabricated with dimensions depending on the materialof the cap layer to give at least 1 kΩ between a component and theground contact.

The invention will now be described by way of example, with reference tothe accompanying drawings, in which:

FIG. 1 is a perspective view of a prior art integrated opto-electronicdevice comprising a DFB laser diode in line with an EA modulator;

FIG. 2 is a perspective view of an integrated opto-electronic deviceaccording to the invention, comprising a DFB laser diode in line with anEA modulator, separated by an isolation region from which a groundingcontact is made to a ground plane;

FIG. 3 is a perspective view of the device of FIG. 2, shown with platedpads affixed to the device; and

FIG. 4 is a circuit diagram of the device of FIGS. 2 and 3, showing howthe grounding contact isolates the two components.

FIG. 1 shows, not to scale, an integrated prior art opto-electronicdevice 1 comprising two components, namely a DFB laser diode 2 and an EAmodulator 4, suitable for use as a transmitter in a high speedfibre-optic link operating at 1.55 μm. Currently, high speed linksoperate at 2.5 or 10 Gbits/s, and bit rates of up to 40 Gbits/s havebeen demonstrated in the laboratory.

The device 1 is grown in wafer form, from an n⁺⁺-InP substrate 6 dopedto around 10¹⁹/cc, on which is grown a 2 μm thick n⁺-InP buffer layer 8doped to around 10¹⁸/cc. The laser diode has anIn_(x)Ga_(1−x)As_(1−y)P_(y) active layer 10 which is about 100 nm to 300nm thick, and this is topped by another buffer layer 12, here a“cladding” layer formed from p⁺-InP. The DFB grating for the laser diodecan be contained in the n⁺-InP buffer layer or in the p⁺-InP cap layer.The active region of the DFB laser and the EA modulator usuallycomprises a multiple quantum well (MQW) structure. The MQW structure isespecially advantageous in the modulator section where the absorptionedge of the modulator can be shifted towards longer wavelengths by theapplication of an electric field (the quantum confined Stark effect).

The output facet 9 of the modulator is anti-reflective coated for goodtransmission through the facet, and the back facet 11 of the laser diodemay be reflectively coated or left uncoated.

The cladding or upper buffer layer 12 is grown to be about 2 μm thick,on top of which a 100 nm to 200 nm thick cap layer is deposited. The caplayer is formed from p⁺⁺-GaInAs, highly doped to around 10¹⁹/cc, inorder to provide a good low resistance ohmic contact for the electricalconnection to the laser diode 2. Then, using well-known fabricationtechnology, the wafer is coated with an oxide layer, here SiO2 (notillustrated) deposited by in a plasma enhanced chemical vapourdeposition (PECVD) process. This oxide layer is photolithographiclypatterned and dry etched to remove the cap layer 16 and all but 200 nmof buffer layer except along a 3 μm wide ridge stripe 14. The ridgestripe 14 therefore rises about 2 μm above the surrounding surface.Finally the PECVD oxide layer is removed from the ridge stripe 14 toexpose again the cap layer 16.

The ridge stripe 14 has the effect of guiding an optical mode 15 alongan active region 17 beneath the ridge stripe 14.

The ridge stripe 14 extends from the laser diode 2 towards the EAmodulator 4 through an isolation region 18. The EA modulator has asimilar structure to that described for the laser diode, except that theabsorption edge of the unbiased modulator is at a shorter wavelength(typically 30 nm to 100 nm shorter) than the gain maximum and theemission wavelength of the laser diode.

The isolation region 18 comprises a gap 20 etched in a process similarto that described above to remove completely the cap layer 16, and ifneeded the top of the upper p⁺-InP buffer layer 12. The etching of thegap 20 stops short of the depth which would cause reflections andinterfere with light guided by the active region 17 extending underneaththe ridge stripe 14. Because of the need to maximise the isolationresistance without adversely affecting the optical properties of theridge waveguide, and also the need to align the photolithographicpattern with the ridge stripe 14 between the components 2,4, thepositioning and etching of the gap is a highly critical process. It isvery difficult to achieve this alignment in a production environment.The isolation region 18 so produced approximately doubles the isolationresistance between the laser diode 2 and modulator 4. Protonimplantation could be used to increase this isolation resistance furtherto 1-10 MΩ.

The cap layer 16, the sides of the ridge stripe 14, and the surroundingupper buffer layer 10 are then coated with a PECVD oxide layer 22, herean SiO₂ layer. This is pattered and etched in a similar process to thatdescribed above, to open up two contact windows on the ridge stripe 14,one 24 above the laser diode and the other 26 above the modulator.

Metal is then vacuum deposited on the device 1 using well knowntechniques in two stages, first with a TiPt layer which is patternedusing a lift-off process, and then final depositing of a TiAu layer,followed by metal wet etch in a photolithographically defined areas. Theremaining TiAu layer forms two contacts 28, 30 which cover over thecontact windows 24, 26 to make good ohmic contacts through the cap layerwith the laser diode 2 and modulator 4. Six other metalised areas 31-36are also formed that do not make any electrical connection but ontowhich pads (not shown) may be plated in order to provide physicalprotection to the ridge stripe 14.

Although not illustrated, the substrate 6 would be soldered onto a heatsink in a conventional manner.

The prior art device 1 is about 700 μm long (ie in the direction of theridge 14) and about 300 μm wide. The lengths of the laser diode 2, gapisolation region 18 and modulator 4 are, respectively about 450 μm, 50μm and 200 μm.

FIGS. 2 and 3 illustrate, not to scale, an integrated opto-electronicdevice 101 according to the invention. This device 101 is similar to theprior art device 1 described above, and so similar features areindicated with reference numerals incremented by 100.

The device 101 has an isolation region 118 that is about 70 μm inlength, and so is slightly longer than the prior art 18 isolationregion. This allows sufficient space for an isolation contact window 140formed to an unbroken cap layer 116 extending between the laser diode102 and the modulator 104. The isolation contact window 140 is formed inthe same manner and at the same time as the contact windows 124,126 forthe components 102,104. This is much more convenient than the formationof the isolation gap described above, avoiding the need to align theisolation region in a separate process step from the alignment of thecontact windows.

In a separate process step prior to etching of the contact and isolationwindows, a ground contact window 142 to the substrate is formed into thedevice, passing through the buffer layers 108,112, active layer 110 andabout 2 μm into the 100 μm thick substrate 106. At this level, thesubstrate is an effective ground plane for the components 102,104. Then,during the deposition of the PEVCD oxide layer 122 over the cap layerand surface either side of the ridge, the sides 144 and base (not shown)of the of the ground window 142 are also covered in the oxide layer. Theoxide covering the base of the ground window 142 is then removed in thesame process step which opens up the contact 124,126 and isolation 140windows.

In one level of fabrication, a TiPt/TiAu conductor 146 is depositedbetween the isolation contact window 140 and the ground contact window142, at the same time as the component conductors 128,130 are deposited.A 10 μm gap separates the conductors 128,130 from the ground conductor146. This method is quite convenient as there are no additional processsteps. It has been observed, however, that for devices created under.particular processing conditions, the junction between the TiPt and then++ substrate, that the junction behaves like a Schottky diode. It istherefore preferred if the portion of the conductor 146 away from theridge stripe 114 is formed in a separate process step from a singlelayer of AuGeNi or AuSn alloy.

Although it would be possible to provide a path to ground other than bydepositing the conductor 146 from the isolation contact window 140 tothe ground contact window 142, for example with a free-standing,grounded wire bonded to the isolation contact window, the integratedconstruction described above is believed to be particularly advantageousbecause it facilitates conduction of very high frequency (of the order1-10 GHz, or even higher) stray currents induced by the modulation.

The device described above is of relatively low integration density.Because of the ample space afforded by the area to the left side (asdrawn) of the ridge, the ground window 142 can be relatively largecompared with the contact 124,126 and isolation 140 windows. In thisexample, the ground window is 50 μm wide (in the direction of the ridgestripe 114) and 100 μm long. The alignment of the ground window withrespect to the other features is therefore not as critical as thealignment of the cap layer gap 20 of the prior art device. Furthermore,the depth of the ground window is not critical in order to achieve agood path to ground in the substrate 106. This embodiment of the deviceis therefore very well suited to a production environment.

Referring now to FIG. 3, an Au contact pad 158 is plated to metalisedarea 128, and Au protective pads 162,163,165,166 are plated to metalisedareas 132,133,135,136. Contact pad 158 is provided only to facilitateinitial testing of the laser diode. After testing, the device may thenbe packaged in an industry standard package (not shown), with a singlemode optical fibre coupled with a spherical lens to the output facet ofthe modulator 104, and with gold bond wires soldered onto metalisedareas 128 and 130

For convenience, the same reference numbers as used above are used inFIG. 4 to refer generally to equivalent circuit elements. FIG. 4 showselectrically how the isolation region 118 helps to isolate the steadylaser diode 102 from electrical disturbance from the modulator 104.

The laser diode is forward biased by a V_(LD) at about 1.6 V, and themodulator is reverse biased with a modulation V_(M), modulated between−0.5 V (transmissive) and −2.0 V (absorptive) at up to 10 Gbit/s, orhigher. In the absence of an isolation feature, a stray current from thelaser diode 102 to the modulator 104, varying between about 0.5 mA and0.9 mA, would cause wavelength or intensity chirping of the laser.

The unbroken cap layer 116 between the laser diode 102 and modulator104, provides a relatively low resistance path R_(i) path to theisolation window, and to the substrate ground 106 through the conductor146. Any stray currents in the cladding or upper buffer layer 112, whichhas a resistivity of 5×10⁻⁴ (about ten times higher than that for thecap layer) will also be drawn toward the cap layer 116 between thecomponents 102,104, and from there to ground 106. In the presentexample, the value of R_(i) is preferred to be about 2-3 kΩ. A value forR_(i) below about 1 kΩ could cause excessive heating of the laser diode,leading to a wavelength shift of the laser diode.

Although the present invention has been described specifically for theexample of a DFB laser diode in line with an EA modulator, the inventionis applicable to any pair or number of opto-electronic componentsmonolithically integrated on a substrate, where stray currents from onecomponent need to be isolated from another component. For example, anoptical waveguide with a split into two waveguides at a Y-junction, mayhave electrically driven or modulated active optical regions in two orthree of the arms of the “Y”, for example an optical amplifier ormodulator. It may then be desirable to provide an isolation region atthe junction of the three arms with which to electrically isolate thetwo or three optically active regions.

Another example of an opto-electronic device where there is a need forelectrical isolation between optically coupled components would betunable DFB laser diodes. These may be formed from two or three in-linesections, with a tunable Bragg grating section adjacent to a steadystate amplification section.

Opto-electronic devices according to the invention provide a convenientand economical means of electrically isolating integratedopto-electronic components. The process steps involved may be similar toother standard steps used in the fabrication of such devices. There isno need for additional expensive processing equipment not used in othersteps, such as ion beam implantation equipment. The tolerances in thealignment of the isolation region, and grounding contacts or windows maybe reduced compared with those of prior art isolation regions.

What is claimed is:
 1. An integrated opto-electronic device, comprisingat least two opto-electronic components fabricated on the samesubstrate, two of the components being: separated by an electricalisolation region; linked optically across the isolation region by awaveguide; and capped by a contact layer through which ohmic contactsare made to operate the components, characterised in that the contactlayer extends through the isolation region from which a groundingcontact is made to ground the contact layer in the isolation region andto electrically isolate the two components from each other.
 2. Anintegrated opto-electronic device as claimed in claim 1, in which thecontact layer extends contiguously between said two components.
 3. Anintegrated opto-electronic device as claimed in claim 1, in which one ofsaid two components is a laser diode, and the other of said twocomponents is a modulator for modulating the output of the laser diode.4. An integrated opto-electronic device as claimed in claim 1, in whichthe contact layer is covered by a passivating layer, the passivatinglayer having contact windows through which the contacts are made.
 5. Anintegrated opto-electronic device as claimed in claim 1, in which thedevice has a ground plane, the grounding contact from the contact layerbeing made to the ground plane.
 6. An integrated opto-electronic deviceas claimed in claim 5, in which a contact window is provided into thedevice for the grounding contact to the ground plane.
 7. An integratedopto-electronic device as claimed in claim 6, in which a passivatinglayer extends over the device, the window into the device extendingthrough said passivating layer.
 8. An integrated opto-electronic deviceas claimed in claim 5, in which at least one of said components isgrounded by the ground plane.
 9. An integrated opto-electronic device asclaimed in claim 1, in which the contacts are each made by a depositedconducting layer.
 10. An integrated opto-electronic device as claimed inclaim 9, in which a contact window is provided into the device for thegrounding contact to the ground plane and wherein the depositedconducting layer covers over one or more windows.
 11. An integratedopto-electronic device as claimed in claim 1, in which the contact layeris a ternary cap layer.
 12. An integrated opto-electronic device,comprising at least two opto-electronic components fabricated on thesame substrate, two of the components being: separated by an electricalisolation region; linked optically across the isolation region by awaveguide; and capped by a contact layer through which ohmic contactsare made to operate the components, characterised in that the contactlayer extends to the isolation region from which a grounding contact ismade to ground the contact layer in the isolation region and toelectrically isolate the two components from each other, in which thecontact layer provides a resistance of at least 1 kΩ between a componentand the ground contact.
 13. An integrated opto-electronic device asclaimed in claim 12, in which the contact layer extends contiguouslybetween said two components.
 14. An integrated opto-electronic device asclaimed in claim 12, in which one of said two components is a laserdiode, and the other of said two components is a modulator formodulating the output of the laser diode.
 15. An integratedopto-electronic device as claimed in claim 12, in which the contactlayer is covered by a passivating layer, the passivating layer havingcontact windows through which the contacts are made.
 16. An integratedopto-electronic device as claimed in claim 12, in which the device has aground plane, the grounding contact from the contact layer being made tothe ground plane.
 17. An integrated opto-electronic device as claimed inclaim 16, in which a contact window is provided into the device for thegrounding contact to the ground plane.
 18. An integrated opto-electronicdevice as claimed in claim 17, in which a passivating layer extends overthe device, the window into the device extending through saidpassivating layer.
 19. An integrated opto-electronic device as claimedin claim 16, in which at least one of said components is grounded by theground plane.
 20. An integrated opto-electronic device as claimed inclaim 12, in which the contacts are each made by a deposited conductinglayer.
 21. An integrated opto-electronic device as claimed in claim 20,in which a contact window is provided into the device for the groundingcontact to the ground plane and wherein the deposited conducting layercovers over one or more windows.
 22. An integrated opto-electronicdevice as claimed in claim 12, in which the contact layer is a ternarycap layer.